Fix for 's' units when setting string max_time

[christopherbunn]

Per Angela's comment on PR #125, this allows 's' units to be used for max_time

[codecov]

Codecov Report

Merging #132 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   94.07%   94.08%   +<.01%     
==========================================
  Files          55       55              
  Lines        1434     1436       +2     
==========================================
+ Hits         1349     1351       +2     
  Misses         85       85

Impacted Files	Coverage Δ
evalml/tests/automl_tests/test_autoclassifier.py	100% <100%> (ø)	⬆️
evalml/utils/convert_time.py	100% <100%> (ø)	⬆️

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[angela97lin]

LGTM!